1. Field of the Invention
The present invention relates to an oxide ion conductor and a method of producing the same. In particular, the present invention relates to an oxide ion conductor which is composed of a composite oxide that exhibits anisotropy in relation to the oxide ion conductivity. The oxide ion conductor is preferably usable as a solid electrolyte of a fuel cell. The present invention also relates to a method of producing the oxide ion conductor.
2. Description of the Related Art
An oxide ion (O2−) conductor is suggested as an electrolyte of the fuel cell. The fuel cell attracts the attention as a low pollution electric power supply source in response to the growing concern about the environment protection in recent years. When the oxide ion conductor is used, the entire fuel cell can be made of solid materials, because the oxide ion conductor is a solid. Therefore, the structure is simple. Further, it is possible to reduce the frequency of the maintenance operation because no liquid leakage occurs.
Thus, Japanese Laid-Open Patent Publication Nos. 8-208333 and 11-130595 suggest oxide ion conductors composed of a composite oxide of a rare earth element and Si. On the other hand, the present applicant-has suggested, in Japanese Laid-Open Patent Publication No. 2002-252005, an oxide ion conductor which is composed of a composite oxide of a rare earth element and Ge and which can be manufactured at a low temperature as compared with the oxide ion conductors as described above. The oxide ion conductor as described above is manufactured by sintering a powder of the oxide of the rare earth element and a powder of silicon oxide or germanium oxide. The oxide ion conductor obtained in this way is the composite oxide, and the crystalline structure thereof is an apatite type structure.
It is preferable that the oxide ion conductor of this type is excellent in oxide ion conductivity. However, it is not easy to remarkably improve the oxide ion conduction. In particular, when it is intended to manufacture single crystals by a method described in Japanese Laid-Open Patent Publication No. 11-130595, a shape of the single crystal is specified to certain ones. Further, it is necessary to melt the material. Therefore, the oxide ion conductor, which is finally obtained, has a narrow composition range.
The present inventors note the mechanism of the oxide ion conduction in the composite oxide during the study for improving the oxide ion conductivity of the composite oxide having the apatite type structure.
The crystal system of the substance having the apatite type structure usually belongs to the hexagonal system. In this case, the oxide ion conduction is caused by the movement of an oxide ion (O2−) present at the 2a site. Accordingly, it can be assumed that the oxide ion conductivity, which is obtained in the movement direction, may be improved if the directions of the movement of O2− are approximately coincident with each other in the respective crystal grains by aligning the directions of orientation of the crystal grains of the composite oxide. In other words, it is considered that anisotropy is brought about in the directions of orientation of the crystals and consequently in the oxide ion conductivity so that ions are moved in the direction in which the oxide ion conductivity is high. However, an oxide ion conductor having the apatite type structure has not been found, in which the directions of orientation of the crystals are approximately coincident with each other. Also, the method of producing such an oxide ion conductor has not been found.
Each of Japanese Laid-Open Patent Publication Nos. 2002-53367 and 2002-193672 suggests a ceramic sintered product having anisotropy and a method of producing the same. A powder is dispersed in a solvent to prepare a slurry, and then the slurry is solidified in a magnetic field to obtain a compact which is thereafter sintered. However, such a technique is used to improve the mechanical characteristics such as strength and toughness in a specified direction, which is not directed to the improvement in the oxide ion conductivity in a specified direction of a composite oxide having the apatite type structure or the like.